Large residential buildings, such as apartment complexes or student dormitories, as well as schools and offices are commonly equipped with central HVAC systems. Common configurations include one-pipe, two-pipe, and four-pipe systems. In all of these configurations, a heating and/or cooling medium that is most commonly water but for heating may also be steam, is centrally heated or cooled and then pumped through pipes throughout the building. The medium then flows through fan-coil unit ventilators where the heat or cold is partially transferred to the air in the rooms to be heated or chilled.
FIG. 1 shows a fan-coil unit ventilator 100 (in the following just ‘fan-coil unit’) for a two-pipe system as it is commonly installed in apartment buildings and similar structures. Its primary components are an electric fan 102 and a coil 104 that exchanges heat between the heating or cooling medium and the air forced through it by the fan 102. The unit is connected to mains power through hot wire 108 and neutral wire 110. A switch 106 installed in the unit allows the user to switch the unit on an off. If switched on, the switch 106 allows electricity to pass through wire 112, to the fan 102, setting it in motion. A central heating and/or cooling plant pumps the heating/cooling medium, typically water, into pipe 114. From there, a junction that may include a balancer valve diverts a certain amount of that water through pipe 118 into the coil 104. If the fan is operating and the water is hot, the air forced through the coil will be heated, and if the water is cold the air will be chilled. Then the water passes back through pipes 120 and 122 to the central plant to be heated or cooled again.
Such two-pipe systems are very popular, especially in older buildings but also in some new structures, because of their low cost of installation. The switch 106, which may also offer high and low settings instead of just on and off, gives the resident of each room a rudimentary level of control over the desired temperature. Alternatively to the two-pipe system shown, several fan-coil unit ventilators may be installed serially with the water exiting from one unit flowing into the next unit. This one-pipe system is not very popular anymore. Instead of the two pipes, one to and one from the heating plant, there may also be four pipes, with separate pipes for cooling and heating media from the plant and separate return pipes for each, which is known as a four-pipe system. The four-pipe system has the advantage over the two-pipe system that some rooms can be cooled while other rooms are being heated, whereas two-pipe systems are restricted to either heating or cooling for the entire building and thus require seasonal or more frequent changeovers between heating and cooling. In four-pipe systems, valves, which can be either manually or automatically actuated, need to shut off the flow of the cooling medium when heating is desired and the flow of the heating medium when cooling is desired.
The temperature control provided by a simple on/off or off/low/high switch is very crude, however. There is no thermostatic control provided. One solution is for users to operate the switch frequently according to the present room temperature, which is inconvenient and impossible to do while a room is not being occupied or the resident is busy or sleeping. The most common solution in practice thus is for the user to set the fan-coil unit to provide more heating or cooling than is desired and getting a more fine-grained control of the temperature by cracking a window open. This solution is grossly wasteful of energy invested for heating and cooling.
Over the past decades, thermostats for residential and commercial applications have made great progress. Traditionally, all a user could do with a thermostat was to set it into either heating and cooling mode, then set a temperature setpoint, and the thermostat would switch the heating or cooling on when the temperature was a certain amount below or above, respectively, the setpoint, and switch it off again when the temperature had increased or decreased, respectively, to a certain amount above or below the setpoint. Even this, of course, would be better than the method of switching a fan-coil unit on and then opening the window to regulate the temperature. Modern thermostats, however, have much more advanced functions. Even cheap thermostats now allow the user to set a schedule with different temperature setpoints for different times of the day. More advanced thermostats can be programmed through the Internet, automatically learn the user's schedule, sense automatically when someone is at home and predict return times, and my own Application 61,863,381 teaches a thermostat that finds a sophisticated optimal control strategy to maximize comfort and minimize cost based upon real-time energy prices, weather predictions, and other data.
These developments in thermostat controls have mostly passed by installations with fan-coil units, especially older installations where a retrofit would be necessary. Part of the reason is that almost all modern residential thermostats are designed to switch a low current of a safe voltage, usually 24 V AC, but fan-coil units need to be switched at higher currents and at mains voltage. There are some thermostats designed to switch mains voltage available, but they are much more expensive than standard low-voltage thermostats and typically offer few or none of the advanced functions of modern thermostats. Mains voltage thermostats are also difficult to install because the connection from the fan-coil unit to the thermostat must be safe for mains voltage and substantial currents, so that installation must be performed by a qualified electrician and often requires tearing walls and floors open to install code-compliant mains wiring. What is more, in a one- or two-pipe system that switches between heating and cooling, most mains voltage thermostats cannot automatically detect the changeover. If, for example, the thermostat is set to cooling but temperatures drop and the HVAC system is changed over to heating, the thermostat will not activate the fan-coil unit as temperatures drop. When temperatures happen to rise far enough that cooling would make sense, the thermostat will turn on the fan-coil unit, which, since the system is operating in heating mode, will raise the temperature even more, and the thermostat becomes stuck in this operating condition. It is, therefore, not surprising that few buildings with fan-coil units have retrofitted thermostats.
There are a few solutions available that enable the installation of a special low-voltage thermostat on a fan-coil unit. These solutions typically consist of a transformer, a printed circuit board containing one or more relays, a temperature sensor, and a specialized thermostat with a connection for the temperature sensor. The transformer and the printed circuit board are then installed in an enclosure in or near the fan-coil unit. Low-voltage signal and power wires are run from the transformer and the relay board to the thermostat and to the temperature sensor. This wiring requires a qualified electrician and is fairly time-consuming. Both the transformer and the printed circuit board must be mounted in a way that is safe and code-compliant for components that have exposed parts under mains voltage. The wiring is complicated and has to connect four new components, the thermostat, the relay board, the transformer, and the temperature sensor. The specialized thermostat will then use the signal from the temperature sensor, which is installed on the pipe 118 bringing heating/cooling medium into the fan-coil unit, to detect changeovers between heating and cooling and change its own mode accordingly. These solutions work, but they are complicated, expensive, and the specialized thermostats required usually offer much less advanced features than advanced standard thermostats for low voltage. Again, it is not surprising that this solution is not widely adopted and in particular that it is not widely retrofitted into existing installations.
US Patent Application 2012,027,3580 teaches a self-configuring low-voltage thermostat that can sense whether it is connected to a one-speed or a two-speed HVAC system, although it does not deal with the problem of mains-powered fan-coil units.
The reader will see that the present situation for controlling the operation of fan-coil units attached to central HVAC systems, in particular for retrofitting existing systems, is quite unsatisfactory. Many installed systems do not have any thermostats installed, and users regulate the temperature through opening windows. This is equally uncomfortable and wasteful. Existing solutions to retrofit thermostats to a fan-coil unit require mains-voltage wiring to the intended thermostat location; this is expensive, laborious, and messy, most mains-voltage thermostats are quite unsophisticated and expensive compared to low-voltage thermostats, and most mains-voltage thermostats don't solve the problem of changing the thermostat mode as a one- or two-pipe HVAC system is being switched over from heating to cooling or vice versa. Some alternative solutions allow for low-voltage wiring from the fan-coil unit to the thermostat and include a sensor to handle changeovers, but are expensive and consist of several components that are difficult to install and expose mains voltage, such as on the soldering points of a printed circuit board holding relays.
None of the existing solutions allow for more advanced communications between the fan-coil unit and the world outside of it. The central HVAC unit is not aware when more fan-coil units come on or go off and thus cannot anticipate their demand for heating or cooling power but has to observe the temperature of the returning heating or cooling water. Existing solutions do not allow to charge residents of each unit for the amount of heat and cooling they consume in order to provide an incentive to conserve energy, and they certainly do not allow the integration of real-time pricing where cooling power is priced differently depending upon the current real-time price of electricity.